What type of Nikkys do you have? I use the wool and breatheable poly, and the cotton, and yes, you can recondition all of them.
I would recommend lanolin melted in boiling water for the wool. Just add the covers, stir and allow to cool.
For the breatheable poly, I would recommend using Nikwax polar proof wash-in waterproofer.
For the cotton, I would recommend Nikwax Cotton Proof wash-in waterproofer.
The nikwax can be found at some sporting goods stores, although the cotton proof is relatively new, and most don't have it yet. I went to the Nikwax site for mine, but I also know that www.stactinator.com
sells it too, if you would prefer to support a WAHM....
Home that helps! Love those Nikkys!!!